1. Field of the Invention
The present invention relates to a combustion engine with an EGR apparatus and particularly relates to a combustion engine with an EGR apparatus having an oxygen concentration sensor mounted in an intake path.
2. Description of the Related Art
As is known in the prior art, an EGR (exhaust gas recirculation) apparatus recirculates a part of an exhaust gas of the combustion engine for controlling the combustion in the combustion engine. An EGR control valve is mounted in an EGR path which recirculates the part of the exhaust gas to the intake path. The opening of the EGR control valve is controlled on the basis of the amount of an EGR gas recirculated to the intake path. Japanese Unexamined Patent Publication (Kokai) No. 57-148048 discloses a method for calculating the amount of EGR gas wherein an air flow meter for detecting the amount of an air introduced from an atmosphere and a pressure detector for detecting the amount of an intake air containing an EGR gas and the air introduced from the atmosphere, are mounted in the intake path, and the amount of EGR gas is calculated on the basis of the amount of the intake air and the amount of the air introduced from the atmosphere.
In order to correctly detect the amount of the intake air drawn into the combustion engine, the detected amount of the intake air must be compensated on the basis of the temperature of the intake air and the temperature of the air introduced from the atmosphere. However, when the temperature of the intake air and the temperature of the air introduced from the atmosphere are detected by temperature sensors, it is difficult to correctly detect the temperatures at each moment because the temperature sensors have relatively large time constants. During the transitional operational condition of the combustion engine wherein the change of the temperature is relatively large, the temperature detected by the temperature sensor greatly differs from the actual temperature, therefore, it is difficult to correctly detect the amount of the EGR gas. If the amount of the EGR gas is not correctly detected, the proper combustion is not achieved in the combustion engine and the amount of NOx and smoke in the exhaust gas increases.
Further, when a plurality of sensors such as the air flow meter, the pressure sensor and the temperature sensor are used, the errors of the values detected by respective sensors accumulate and, therefore, the errors of the amount of the intake air and the air introduced from the atmosphere accumulate, the amount of the intake air and the air introduced from the atmosphere being calculated on the basis of the values detected by respective sensors. Further, when a plurality of sensors are used, the complexity of a control system of the EGR apparatus increases, therefore, the cost of the EGR apparatus increases.
When a quick-response temperature sensor whose time constant is relatively small is used, it is possible to detect the temperature which is relatively close to the actual temperature, by means of the quick-response temperature sensor during the transitional operational condition of the combustion engine. However, since the quick-response temperature sensor is expensive, the cost of the EGR apparatus increases.
As is well known, in order to judge the EGR apparatus to be correct or wrong, i.e., in order to judge the failure of the EGR apparatus, an EGR ratio is used, the EGR ratio being a ratio of the amount of the EGR gas to the amount of the intake air. However, when the amount of the EGR gas in not correctly detected, the EGR ratio on the basis of the amount of the EGR gas is also incorrect, therefore, it is impossible to correctly judge the failure of the EGR apparatus.